60 Watt switching Power supply

Posted on Sep 24, 2012

This paper gives a practical example of the design of an off-line switching power supply. Factors governing the choice of a discontinuous flyback topology are discussed. The design uses a pulsed-width modulation (PWM) control scheme implemented with a Unitrode UC3840 IC. This chip's voltage-feed-forward feature is used to achieve improved output regulation. The paper discusses closing the control loop to achieve both stability and adequate dynamic regulation, and provides guidelines for transformer design and component selection.

60 Watt switching Power supply
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The circuit developed herein operates from a l17V (::!::15%), 60 Hz line and meets the following objectives: I. Output voltages: a. +5V, :!::5%: 2.5A-5A Ripple voltage: 50mV p-p maximum b. +12V, :!::3%: IA-2.9A Ripple voltage: lOOmV p-p maximum 2. Fificiency: 70% minimum 3. Line isolation: 3750V These objectives are met by using a flyback converter topology with a MOSFET power switch operating at 80kHz. The design features primary side control. I. SELECTION OF FLYBACK TOPOLOGY The flyback, when compared to other switching regulator topologies, has several cost and performance advantages: Cost Advantages: 1. For output power levels less than -150W, the design of the power transformer (coupled inductor) is relatively simple. 2. Assembly costs for the flyback regulator are low due to a low overall component count. In particular, only one magnetic element (i.e., the transformer) is employed as no inductors are used in the output filters. 3. Output rectifier BY requirements are low, since they do not need to block voltages which in other topologies are developed across the fIlter inductor.

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